1. Field of the Invention
This invention generally relates to testing electrical circuits, and more particularly to a system and method for detecting and classifying defects in an electrical circuit during or after a manufacturing process. The present invention is also a system and method for identifying one or more process problems that caused the defects detected during a test.
2. Description of the Related Art
Because of their small size and superior performance, thin-film-transistor (TFT) arrays have evolved as a preferred technology for a variety of applications including but not limited to flat-panel LCD displays and imaging and sensing systems used in consumer electronics.
During the manufacturing process, defects may develop which, is left unaddressed, may diminish the performance of the array. These defects include electrical shorts between the gate and common lines connecting the transistors and their associated storage elements. The need to test for defects becomes more important as the number of transistors in the array increases. This may be attributable to several factors. One is that the probability of a short developing tends to vary linearly with the length of the gate and common lines. The number of these lines and their proximity to one another also plays a role in increasing the chances of a short occurring. For example, in a double-gate or double-common-line configuration, the gap between the gate and common lines is likely to be narrower than in single-gate-line and single-common-line layouts. The likelihood of a short developing consequently increases.
Once a short has been located in a TFT array, it can be repaired by cutting the short. Existing methods for locating shorts and other defects in transistor arrays, however, have proven to be inaccurate. This is especially true of shorts between the gate and common lines of the array, as this type of defect does not give off a distinctive signal at the affected pixel location which can be detected by existing methods. As a result, the defect may never be located or at best may only be detected to lie within a certain general area which includes other pixel elements that are properly functioning. Because of this imprecision, the defect may not be able to be corrected because it cannot be located with any degree of accuracy. In a worst case, an attempt to eliminate the defect may result in destroying a properly functioning portion of the array, thereby compounding the problem and in many cases tendering the transistor array unusable for all intents and purposes.
In view of the foregoing considerations, it is apparent that there is a need for a system and method for, first, detecting the existence of a defect in a thin-film-transistor array and, second, accurately detecting a location of the defect so that corrective action may be taken without disturbing other portions of the array that are properly functioning.